discontinuity

the point or the value of the variable at which a curve or function becomes discontinuous

4.

(geology)

a zone within the earth where a sudden change in physical properties, such as the velocity of earthquake waves, occurs. Such a zone marks the boundary between the different layers of the earth, as between the core and mantle See also Mohorovičić discontinuity

A usually uneven surface between two layers of rock or sediment that represents either an interruption in the deposition of the layers, as in an unconformity, or a displacement of one or both layers relative to each other, as in a fault.

A surface within the Earth across which the velocities of seismic waves change. The discontinuities are located at the boundaries between the Earth's various layers and correspond to changes in the elastic properties of the Earth's materials.